What causes garden pond water to go green?

Green water in your garden pond is caused by an algae with the botanical name Chlamydomonas  (algae is actually a diverse group of aquatic organisms). Some are familiar to most of us like, Blanket weed with the botanical name Cladophora. Algae is mainly caused by excessive nutrients (over feeding) and sunshine (not enough shade) I will show you how to clear green water! Without using chemicals,


Two types of algae you can clear from a pond, without using chemicals;

  1. Green water (Chlamydomonas) 
  2. Blanket weed (Cladophora)
nature pond with blanket weed
nature pond with blanket weed
pond with green water
pond with green water

What to do when your pond water is green.

What problems do algae cause in a garden pond?

  1. Green water algae is harmless to fish and other pond-life. However, if not controlled and just left to multiply. The Algae will turn the pond water green, looking unpleasant and making it difficult to see your fish, newts, frogs and other pond-life and aquatic plants, spoiling the enjoyment of having a pond. It can also be embarrassing when showing of your pond.
  2. Blanket weed if left unchecked can cause serious problems in a pond, depleting oxygen and is dangerous to fish and frogs, etc. trapping them in its slimy growth and it can also smother aquatic plants. In moderation, algae can provide great habitat for aquatic insects.

 The main cause for both green water algae and blanket weed is high fertility from excessive nutrients, mainly from fish poo (especially Koi who seem to spend all their time eating and pooing), uneaten fish food, organic debris (rotting pond plants and leaves), fertiliser run-off into the pond from higher ground and fertilisers added to plants you have bought from your garden centre, insufficient shade in very sunny sites (algae thrives on sunshine). Some algae is natural in a newly built and filled pond and will clear if you follow the tips below.


An algae attack is normally worse in spring and summer, during sunny weather. 


So how do I get rid of it without using chemicals?

How to stop pond water going green without using chemicals?

The water in my nature pond was clear apart from a few months after building and filling it with tap water, always has been clear. So what's my answer? It's simple, follow these good pond-keeping tips and don't use chemicals.

The algae that causes green water cannot be filtered out it is a micro-organism. Therefore, it must be starved out. 


Don't overstock with fish, avoid Koi completely in a wildlife pond, they eat everything in sight including the wildlife. And Poo it all out, thus feeding the algae.


Never overfeed, once the pond culture is established there is no need to feed goldfish anyway. If feeding the fish is part of the enjoyment you and the kids, or in my case grandchildren, get from having a pond in your garden. Follow the instructions on the packet and deduct about 30%. Don't overfeed.


Avoid letting fertilisers get into the water, from run-off in your garden or from pond plants you have bought to plant. It is a good ideal to remove plants from pots, tip the contents of the pot onto flower beds or the compost heap. Then, run the plant under a tap to rinse off any fertilised soil and plant into the substrate of the pond. Cover the root with a rock to stop the plant floating off.


Remove excess rotting pond plants and fallen leaves, etc. I say excess because in a nature pond some organic debris should be allowed to sink to the bottom to form the substrate on the bottom of the pond. This will be the basis for a stable pond environment. The micro-organisms which will take care of clear pond water need some subsoil to live in.

Cover at least half of the surface with floating or big leafed plants like water lilies. These plants compete with algae for light and so can make a big difference to reduce the levels of sunlight reaching the water.


Don't completely shade a nature pond all living things need light)

Controlling and removing Blanket weed in a garden pond


Very difficult to completely stop Blanket weed appearing in even the best maintained garden ponds. The spores can be brought to your pond by insects, birds or on pond plants you add to your pond. You can't stop it, but you can control Blanket weed.


All the controls we have discussed in the first part of this article (to stop the water going green) will control blanket weed because it thrives on the same nutrients your pond waste provides. Everything from fish waste and uneaten food, to rotting pond plants and fallen leaves.

Here's a reminder;

  • Don't overstock with fish
  • Never overfeed
  • Avoid letting fertilisers get into the water, 
  • Remove excess rotting pond plants and fallen leaves,
  • Cover at least half of the surface with floating or big leafed plants.
  • Never use chemicals. 

What's good about Blanket weed?  It is eating-up the excess nutrients and actually cleaning and purifying your pond so don't panic when you first see it. Leave it growing until you feel it's time to hook it out and add it to your compost heap.

What's bad about Blanket weed? Never leave Blanket weed to cover the pond, it will stifle the plants, trap fish and eventually shut out all sunlight. Killing the pond!

The easiest way to get rid of blanket weed from your pond is to regularly remove it by poking and twirling it round a stick. Or use a. Blanket weed brush  Leave it on the bank close to the pond for a couple of days for any pond-life to return to the water and then add it to the compost heap. 

Oxygenating and aerating the water. I'm not going into a lot of technical stuff here, but a pond needs oxygen to live and also needs to release the gases produced by bacteria breaking down organic matter in the bottom of the pond.  Running water through a stream or over a waterfall with the pump placed at the end of the pond furthest away from the waterfall is the best way to release harmful gasses and ensure there is plenty of oxygen to be easily absorbed into the water. 
Oxygenating plants. In a new pond, stock Ceratophyllum demersum (Hornwort) at a rate of rate of three bunches per m2 and allow it to grow until about a third of your pond is filled, then in summer remove the yellowing stems leaving the dark green/olive brown stems. The bulk of Hornwort will drop down to the bottom of the pond in winter to protect itself from frost and ice. 
Never use chemicals to try to clear green pond water they can be expensive and cannot solve the underlying problem anyway, so can't provide lasting protection. The underlying problems can only be solved long-term by following the good pond-keeping guidelines above. 

In Summary there's a variety of things to do and not to do that can help deal with green water, all aimed at starving out the algae that causes it. Mostly natural and easy to do without using chemicals, the methods include

  • avoiding fertiliser,
  • removing decaying matter,
  • not over-stocking with fish,
  • oxygenating and aerating the water,
  • avoiding chemicals, 
  • sensible shade planting and keeping a healthy garden.  

This Blanket weed brush helps remove the weed,

  • Telescopic brush for removing blanket weed from your pond.
  • Length of the handle can be adjusted from 60 cm to 180 cm.
  • Durable nylon head for collecting thread algae
  • Package dimensions (L x W x H): 94.4 x 18.2 x 4.0 cm.